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Linux-2.6.12-rc2

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Initial git repository build. I'm not bothering with the full history,
even though we have it. We can create a separate "historical" git
archive of that later if we want to, and in the meantime it's about
3.2GB when imported into git - space that would just make the early
git days unnecessarily complicated, when we don't have a lot of good
infrastructure for it.

Let it rip!
This commit is contained in:
Linus Torvalds
2005-04-16 15:20:36 -07:00
commit 1da177e4c3
17291 changed files with 6718755 additions and 0 deletions
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102
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Driver Binding
Driver binding is the process of associating a device with a device
driver that can control it. Bus drivers have typically handled this
because there have been bus-specific structures to represent the
devices and the drivers. With generic device and device driver
structures, most of the binding can take place using common code.
Bus
~~~
The bus type structure contains a list of all devices that are on that bus
type in the system. When device_register is called for a device, it is
inserted into the end of this list. The bus object also contains a
list of all drivers of that bus type. When driver_register is called
for a driver, it is inserted at the end of this list. These are the
two events which trigger driver binding.
device_register
~~~~~~~~~~~~~~~
When a new device is added, the bus's list of drivers is iterated over
to find one that supports it. In order to determine that, the device
ID of the device must match one of the device IDs that the driver
supports. The format and semantics for comparing IDs is bus-specific.
Instead of trying to derive a complex state machine and matching
algorithm, it is up to the bus driver to provide a callback to compare
a device against the IDs of a driver. The bus returns 1 if a match was
found; 0 otherwise.
int match(struct device * dev, struct device_driver * drv);
If a match is found, the device's driver field is set to the driver
and the driver's probe callback is called. This gives the driver a
chance to verify that it really does support the hardware, and that
it's in a working state.
Device Class
~~~~~~~~~~~~
Upon the successful completion of probe, the device is registered with
the class to which it belongs. Device drivers belong to one and only one
class, and that is set in the driver's devclass field.
devclass_add_device is called to enumerate the device within the class
and actually register it with the class, which happens with the
class's register_dev callback.
NOTE: The device class structures and core routines to manipulate them
are not in the mainline kernel, so the discussion is still a bit
speculative.
Driver
~~~~~~
When a driver is attached to a device, the device is inserted into the
driver's list of devices.
sysfs
~~~~~
A symlink is created in the bus's 'devices' directory that points to
the device's directory in the physical hierarchy.
A symlink is created in the driver's 'devices' directory that points
to the device's directory in the physical hierarchy.
A directory for the device is created in the class's directory. A
symlink is created in that directory that points to the device's
physical location in the sysfs tree.
A symlink can be created (though this isn't done yet) in the device's
physical directory to either its class directory, or the class's
top-level directory. One can also be created to point to its driver's
directory also.
driver_register
~~~~~~~~~~~~~~~
The process is almost identical for when a new driver is added.
The bus's list of devices is iterated over to find a match. Devices
that already have a driver are skipped. All the devices are iterated
over, to bind as many devices as possible to the driver.
Removal
~~~~~~~
When a device is removed, the reference count for it will eventually
go to 0. When it does, the remove callback of the driver is called. It
is removed from the driver's list of devices and the reference count
of the driver is decremented. All symlinks between the two are removed.
When a driver is removed, the list of devices that it supports is
iterated over, and the driver's remove callback is called for each
one. The device is removed from that list and the symlinks removed.

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Bus Types
Definition
~~~~~~~~~~
struct bus_type {
char * name;
struct subsystem subsys;
struct kset drivers;
struct kset devices;
struct bus_attribute * bus_attrs;
struct device_attribute * dev_attrs;
struct driver_attribute * drv_attrs;
int (*match)(struct device * dev, struct device_driver * drv);
int (*hotplug) (struct device *dev, char **envp,
int num_envp, char *buffer, int buffer_size);
int (*suspend)(struct device * dev, u32 state);
int (*resume)(struct device * dev);
};
int bus_register(struct bus_type * bus);
Declaration
~~~~~~~~~~~
Each bus type in the kernel (PCI, USB, etc) should declare one static
object of this type. They must initialize the name field, and may
optionally initialize the match callback.
struct bus_type pci_bus_type = {
.name = "pci",
.match = pci_bus_match,
};
The structure should be exported to drivers in a header file:
extern struct bus_type pci_bus_type;
Registration
~~~~~~~~~~~~
When a bus driver is initialized, it calls bus_register. This
initializes the rest of the fields in the bus object and inserts it
into a global list of bus types. Once the bus object is registered,
the fields in it are usable by the bus driver.
Callbacks
~~~~~~~~~
match(): Attaching Drivers to Devices
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The format of device ID structures and the semantics for comparing
them are inherently bus-specific. Drivers typically declare an array
of device IDs of devices they support that reside in a bus-specific
driver structure.
The purpose of the match callback is provide the bus an opportunity to
determine if a particular driver supports a particular device by
comparing the device IDs the driver supports with the device ID of a
particular device, without sacrificing bus-specific functionality or
type-safety.
When a driver is registered with the bus, the bus's list of devices is
iterated over, and the match callback is called for each device that
does not have a driver associated with it.
Device and Driver Lists
~~~~~~~~~~~~~~~~~~~~~~~
The lists of devices and drivers are intended to replace the local
lists that many buses keep. They are lists of struct devices and
struct device_drivers, respectively. Bus drivers are free to use the
lists as they please, but conversion to the bus-specific type may be
necessary.
The LDM core provides helper functions for iterating over each list.
int bus_for_each_dev(struct bus_type * bus, struct device * start, void * data,
int (*fn)(struct device *, void *));
int bus_for_each_drv(struct bus_type * bus, struct device_driver * start,
void * data, int (*fn)(struct device_driver *, void *));
These helpers iterate over the respective list, and call the callback
for each device or driver in the list. All list accesses are
synchronized by taking the bus's lock (read currently). The reference
count on each object in the list is incremented before the callback is
called; it is decremented after the next object has been obtained. The
lock is not held when calling the callback.
sysfs
~~~~~~~~
There is a top-level directory named 'bus'.
Each bus gets a directory in the bus directory, along with two default
directories:
/sys/bus/pci/
|-- devices
`-- drivers
Drivers registered with the bus get a directory in the bus's drivers
directory:
/sys/bus/pci/
|-- devices
`-- drivers
|-- Intel ICH
|-- Intel ICH Joystick
|-- agpgart
`-- e100
Each device that is discovered on a bus of that type gets a symlink in
the bus's devices directory to the device's directory in the physical
hierarchy:
/sys/bus/pci/
|-- devices
| |-- 00:00.0 -> ../../../root/pci0/00:00.0
| |-- 00:01.0 -> ../../../root/pci0/00:01.0
| `-- 00:02.0 -> ../../../root/pci0/00:02.0
`-- drivers
Exporting Attributes
~~~~~~~~~~~~~~~~~~~~
struct bus_attribute {
struct attribute attr;
ssize_t (*show)(struct bus_type *, char * buf);
ssize_t (*store)(struct bus_type *, const char * buf, size_t count);
};
Bus drivers can export attributes using the BUS_ATTR macro that works
similarly to the DEVICE_ATTR macro for devices. For example, a definition
like this:
static BUS_ATTR(debug,0644,show_debug,store_debug);
is equivalent to declaring:
static bus_attribute bus_attr_debug;
This can then be used to add and remove the attribute from the bus's
sysfs directory using:
int bus_create_file(struct bus_type *, struct bus_attribute *);
void bus_remove_file(struct bus_type *, struct bus_attribute *);

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Device Classes
Introduction
~~~~~~~~~~~~
A device class describes a type of device, like an audio or network
device. The following device classes have been identified:
<Insert List of Device Classes Here>
Each device class defines a set of semantics and a programming interface
that devices of that class adhere to. Device drivers are the
implemention of that programming interface for a particular device on
a particular bus.
Device classes are agnostic with respect to what bus a device resides
on.
Programming Interface
~~~~~~~~~~~~~~~~~~~~~
The device class structure looks like:
typedef int (*devclass_add)(struct device *);
typedef void (*devclass_remove)(struct device *);
struct device_class {
char * name;
rwlock_t lock;
u32 devnum;
struct list_head node;
struct list_head drivers;
struct list_head intf_list;
struct driver_dir_entry dir;
struct driver_dir_entry device_dir;
struct driver_dir_entry driver_dir;
devclass_add add_device;
devclass_remove remove_device;
};
A typical device class definition would look like:
struct device_class input_devclass = {
.name = "input",
.add_device = input_add_device,
.remove_device = input_remove_device,
};
Each device class structure should be exported in a header file so it
can be used by drivers, extensions and interfaces.
Device classes are registered and unregistered with the core using:
int devclass_register(struct device_class * cls);
void devclass_unregister(struct device_class * cls);
Devices
~~~~~~~
As devices are bound to drivers, they are added to the device class
that the driver belongs to. Before the driver model core, this would
typically happen during the driver's probe() callback, once the device
has been initialized. It now happens after the probe() callback
finishes from the core.
The device is enumerated in the class. Each time a device is added to
the class, the class's devnum field is incremented and assigned to the
device. The field is never decremented, so if the device is removed
from the class and re-added, it will receive a different enumerated
value.
The class is allowed to create a class-specific structure for the
device and store it in the device's class_data pointer.
There is no list of devices in the device class. Each driver has a
list of devices that it supports. The device class has a list of
drivers of that particular class. To access all of the devices in the
class, iterate over the device lists of each driver in the class.
Device Drivers
~~~~~~~~~~~~~~
Device drivers are added to device classes when they are registered
with the core. A driver specifies the class it belongs to by setting
the struct device_driver::devclass field.
sysfs directory structure
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
There is a top-level sysfs directory named 'class'.
Each class gets a directory in the class directory, along with two
default subdirectories:
class/
`-- input
|-- devices
`-- drivers
Drivers registered with the class get a symlink in the drivers/ directory
that points to the driver's directory (under its bus directory):
class/
`-- input
|-- devices
`-- drivers
`-- usb:usb_mouse -> ../../../bus/drivers/usb_mouse/
Each device gets a symlink in the devices/ directory that points to the
device's directory in the physical hierarchy:
class/
`-- input
|-- devices
| `-- 1 -> ../../../root/pci0/00:1f.0/usb_bus/00:1f.2-1:0/
`-- drivers
Exporting Attributes
~~~~~~~~~~~~~~~~~~~~
struct devclass_attribute {
struct attribute attr;
ssize_t (*show)(struct device_class *, char * buf, size_t count, loff_t off);
ssize_t (*store)(struct device_class *, const char * buf, size_t count, loff_t off);
};
Class drivers can export attributes using the DEVCLASS_ATTR macro that works
similarly to the DEVICE_ATTR macro for devices. For example, a definition
like this:
static DEVCLASS_ATTR(debug,0644,show_debug,store_debug);
is equivalent to declaring:
static devclass_attribute devclass_attr_debug;
The bus driver can add and remove the attribute from the class's
sysfs directory using:
int devclass_create_file(struct device_class *, struct devclass_attribute *);
void devclass_remove_file(struct device_class *, struct devclass_attribute *);
In the example above, the file will be named 'debug' in placed in the
class's directory in sysfs.
Interfaces
~~~~~~~~~~
There may exist multiple mechanisms for accessing the same device of a
particular class type. Device interfaces describe these mechanisms.
When a device is added to a device class, the core attempts to add it
to every interface that is registered with the device class.

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The Basic Device Structure
~~~~~~~~~~~~~~~~~~~~~~~~~~
struct device {
struct list_head g_list;
struct list_head node;
struct list_head bus_list;
struct list_head driver_list;
struct list_head intf_list;
struct list_head children;
struct device * parent;
char name[DEVICE_NAME_SIZE];
char bus_id[BUS_ID_SIZE];
spinlock_t lock;
atomic_t refcount;
struct bus_type * bus;
struct driver_dir_entry dir;
u32 class_num;
struct device_driver *driver;
void *driver_data;
void *platform_data;
u32 current_state;
unsigned char *saved_state;
void (*release)(struct device * dev);
};
Fields
~~~~~~
g_list: Node in the global device list.
node: Node in device's parent's children list.
bus_list: Node in device's bus's devices list.
driver_list: Node in device's driver's devices list.
intf_list: List of intf_data. There is one structure allocated for
each interface that the device supports.
children: List of child devices.
parent: *** FIXME ***
name: ASCII description of device.
Example: " 3Com Corporation 3c905 100BaseTX [Boomerang]"
bus_id: ASCII representation of device's bus position. This
field should be a name unique across all devices on the
bus type the device belongs to.
Example: PCI bus_ids are in the form of
<bus number>:<slot number>.<function number>
This name is unique across all PCI devices in the system.
lock: Spinlock for the device.
refcount: Reference count on the device.
bus: Pointer to struct bus_type that device belongs to.
dir: Device's sysfs directory.
class_num: Class-enumerated value of the device.
driver: Pointer to struct device_driver that controls the device.
driver_data: Driver-specific data.
platform_data: Platform data specific to the device.
current_state: Current power state of the device.
saved_state: Pointer to saved state of the device. This is usable by
the device driver controlling the device.
release: Callback to free the device after all references have
gone away. This should be set by the allocator of the
device (i.e. the bus driver that discovered the device).
Programming Interface
~~~~~~~~~~~~~~~~~~~~~
The bus driver that discovers the device uses this to register the
device with the core:
int device_register(struct device * dev);
The bus should initialize the following fields:
- parent
- name
- bus_id
- bus
A device is removed from the core when its reference count goes to
0. The reference count can be adjusted using:
struct device * get_device(struct device * dev);
void put_device(struct device * dev);
get_device() will return a pointer to the struct device passed to it
if the reference is not already 0 (if it's in the process of being
removed already).
A driver can access the lock in the device structure using:
void lock_device(struct device * dev);
void unlock_device(struct device * dev);
Attributes
~~~~~~~~~~
struct device_attribute {
struct attribute attr;
ssize_t (*show)(struct device * dev, char * buf, size_t count, loff_t off);
ssize_t (*store)(struct device * dev, const char * buf, size_t count, loff_t off);
};
Attributes of devices can be exported via drivers using a simple
procfs-like interface.
Please see Documentation/filesystems/sysfs.txt for more information
on how sysfs works.
Attributes are declared using a macro called DEVICE_ATTR:
#define DEVICE_ATTR(name,mode,show,store)
Example:
DEVICE_ATTR(power,0644,show_power,store_power);
This declares a structure of type struct device_attribute named
'dev_attr_power'. This can then be added and removed to the device's
directory using:
int device_create_file(struct device *device, struct device_attribute * entry);
void device_remove_file(struct device * dev, struct device_attribute * attr);
Example:
device_create_file(dev,&dev_attr_power);
device_remove_file(dev,&dev_attr_power);
The file name will be 'power' with a mode of 0644 (-rw-r--r--).

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Device Drivers
struct device_driver {
char * name;
struct bus_type * bus;
rwlock_t lock;
atomic_t refcount;
list_t bus_list;
list_t devices;
struct driver_dir_entry dir;
int (*probe) (struct device * dev);
int (*remove) (struct device * dev);
int (*suspend) (struct device * dev, u32 state, u32 level);
int (*resume) (struct device * dev, u32 level);
void (*release) (struct device_driver * drv);
};
Allocation
~~~~~~~~~~
Device drivers are statically allocated structures. Though there may
be multiple devices in a system that a driver supports, struct
device_driver represents the driver as a whole (not a particular
device instance).
Initialization
~~~~~~~~~~~~~~
The driver must initialize at least the name and bus fields. It should
also initialize the devclass field (when it arrives), so it may obtain
the proper linkage internally. It should also initialize as many of
the callbacks as possible, though each is optional.
Declaration
~~~~~~~~~~~
As stated above, struct device_driver objects are statically
allocated. Below is an example declaration of the eepro100
driver. This declaration is hypothetical only; it relies on the driver
being converted completely to the new model.
static struct device_driver eepro100_driver = {
.name = "eepro100",
.bus = &pci_bus_type,
.devclass = &ethernet_devclass, /* when it's implemented */
.probe = eepro100_probe,
.remove = eepro100_remove,
.suspend = eepro100_suspend,
.resume = eepro100_resume,
};
Most drivers will not be able to be converted completely to the new
model because the bus they belong to has a bus-specific structure with
bus-specific fields that cannot be generalized.
The most common example of this are device ID structures. A driver
typically defines an array of device IDs that it supports. The format
of these structures and the semantics for comparing device IDs are
completely bus-specific. Defining them as bus-specific entities would
sacrifice type-safety, so we keep bus-specific structures around.
Bus-specific drivers should include a generic struct device_driver in
the definition of the bus-specific driver. Like this:
struct pci_driver {
const struct pci_device_id *id_table;
struct device_driver driver;
};
A definition that included bus-specific fields would look like
(using the eepro100 driver again):
static struct pci_driver eepro100_driver = {
.id_table = eepro100_pci_tbl,
.driver = {
.name = "eepro100",
.bus = &pci_bus_type,
.devclass = &ethernet_devclass, /* when it's implemented */
.probe = eepro100_probe,
.remove = eepro100_remove,
.suspend = eepro100_suspend,
.resume = eepro100_resume,
},
};
Some may find the syntax of embedded struct initialization awkward or
even a bit ugly. So far, it's the best way we've found to do what we want...
Registration
~~~~~~~~~~~~
int driver_register(struct device_driver * drv);
The driver registers the structure on startup. For drivers that have
no bus-specific fields (i.e. don't have a bus-specific driver
structure), they would use driver_register and pass a pointer to their
struct device_driver object.
Most drivers, however, will have a bus-specific structure and will
need to register with the bus using something like pci_driver_register.
It is important that drivers register their driver structure as early as
possible. Registration with the core initializes several fields in the
struct device_driver object, including the reference count and the
lock. These fields are assumed to be valid at all times and may be
used by the device model core or the bus driver.
Transition Bus Drivers
~~~~~~~~~~~~~~~~~~~~~~
By defining wrapper functions, the transition to the new model can be
made easier. Drivers can ignore the generic structure altogether and
let the bus wrapper fill in the fields. For the callbacks, the bus can
define generic callbacks that forward the call to the bus-specific
callbacks of the drivers.
This solution is intended to be only temporary. In order to get class
information in the driver, the drivers must be modified anyway. Since
converting drivers to the new model should reduce some infrastructural
complexity and code size, it is recommended that they are converted as
class information is added.
Access
~~~~~~
Once the object has been registered, it may access the common fields of
the object, like the lock and the list of devices.
int driver_for_each_dev(struct device_driver * drv, void * data,
int (*callback)(struct device * dev, void * data));
The devices field is a list of all the devices that have been bound to
the driver. The LDM core provides a helper function to operate on all
the devices a driver controls. This helper locks the driver on each
node access, and does proper reference counting on each device as it
accesses it.
sysfs
~~~~~
When a driver is registered, a sysfs directory is created in its
bus's directory. In this directory, the driver can export an interface
to userspace to control operation of the driver on a global basis;
e.g. toggling debugging output in the driver.
A future feature of this directory will be a 'devices' directory. This
directory will contain symlinks to the directories of devices it
supports.
Callbacks
~~~~~~~~~
int (*probe) (struct device * dev);
probe is called to verify the existence of a certain type of
hardware. This is called during the driver binding process, after the
bus has verified that the device ID of a device matches one of the
device IDs supported by the driver.
This callback only verifies that there actually is supported hardware
present. It may allocate a driver-specific structure, but it should
not do any initialization of the hardware itself. The device-specific
structure may be stored in the device's driver_data field.
int (*init) (struct device * dev);
init is called during the binding stage. It is called after probe has
successfully returned and the device has been registered with its
class. It is responsible for initializing the hardware.
int (*remove) (struct device * dev);
remove is called to dissociate a driver with a device. This may be
called if a device is physically removed from the system, if the
driver module is being unloaded, or during a reboot sequence.
It is up to the driver to determine if the device is present or
not. It should free any resources allocated specifically for the
device; i.e. anything in the device's driver_data field.
If the device is still present, it should quiesce the device and place
it into a supported low-power state.
int (*suspend) (struct device * dev, u32 state, u32 level);
suspend is called to put the device in a low power state. There are
several stages to successfully suspending a device, which is denoted in
the @level parameter. Breaking the suspend transition into several
stages affords the platform flexibility in performing device power
management based on the requirements of the system and the
user-defined policy.
SUSPEND_NOTIFY notifies the device that a suspend transition is about
to happen. This happens on system power state transitions to verify
that all devices can successfully suspend.
A driver may choose to fail on this call, which should cause the
entire suspend transition to fail. A driver should fail only if it
knows that the device will not be able to be resumed properly when the
system wakes up again. It could also fail if it somehow determines it
is in the middle of an operation too important to stop.
SUSPEND_DISABLE tells the device to stop I/O transactions. When it
stops transactions, or what it should do with unfinished transactions
is a policy of the driver. After this call, the driver should not
accept any other I/O requests.
SUSPEND_SAVE_STATE tells the device to save the context of the
hardware. This includes any bus-specific hardware state and
device-specific hardware state. A pointer to this saved state can be
stored in the device's saved_state field.
SUSPEND_POWER_DOWN tells the driver to place the device in the low
power state requested.
Whether suspend is called with a given level is a policy of the
platform. Some levels may be omitted; drivers must not assume the
reception of any level. However, all levels must be called in the
order above; i.e. notification will always come before disabling;
disabling the device will come before suspending the device.
All calls are made with interrupts enabled, except for the
SUSPEND_POWER_DOWN level.
int (*resume) (struct device * dev, u32 level);
Resume is used to bring a device back from a low power state. Like the
suspend transition, it happens in several stages.
RESUME_POWER_ON tells the driver to set the power state to the state
before the suspend call (The device could have already been in a low
power state before the suspend call to put in a lower power state).
RESUME_RESTORE_STATE tells the driver to restore the state saved by
the SUSPEND_SAVE_STATE suspend call.
RESUME_ENABLE tells the driver to start accepting I/O transactions
again. Depending on driver policy, the device may already have pending
I/O requests.
RESUME_POWER_ON is called with interrupts disabled. The other resume
levels are called with interrupts enabled.
As with the various suspend stages, the driver must not assume that
any other resume calls have been or will be made. Each call should be
self-contained and not dependent on any external state.
Attributes
~~~~~~~~~~
struct driver_attribute {
struct attribute attr;
ssize_t (*show)(struct device_driver *, char * buf, size_t count, loff_t off);
ssize_t (*store)(struct device_driver *, const char * buf, size_t count, loff_t off);
};
Device drivers can export attributes via their sysfs directories.
Drivers can declare attributes using a DRIVER_ATTR macro that works
identically to the DEVICE_ATTR macro.
Example:
DRIVER_ATTR(debug,0644,show_debug,store_debug);
This is equivalent to declaring:
struct driver_attribute driver_attr_debug;
This can then be used to add and remove the attribute from the
driver's directory using:
int driver_create_file(struct device_driver *, struct driver_attribute *);
void driver_remove_file(struct device_driver *, struct driver_attribute *);

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Device Interfaces
Introduction
~~~~~~~~~~~~
Device interfaces are the logical interfaces of device classes that correlate
directly to userspace interfaces, like device nodes.
Each device class may have multiple interfaces through which you can
access the same device. An input device may support the mouse interface,
the 'evdev' interface, and the touchscreen interface. A SCSI disk would
support the disk interface, the SCSI generic interface, and possibly a raw
device interface.
Device interfaces are registered with the class they belong to. As devices
are added to the class, they are added to each interface registered with
the class. The interface is responsible for determining whether the device
supports the interface or not.
Programming Interface
~~~~~~~~~~~~~~~~~~~~~
struct device_interface {
char * name;
rwlock_t lock;
u32 devnum;
struct device_class * devclass;
struct list_head node;
struct driver_dir_entry dir;
int (*add_device)(struct device *);
int (*add_device)(struct intf_data *);
};
int interface_register(struct device_interface *);
void interface_unregister(struct device_interface *);
An interface must specify the device class it belongs to. It is added
to that class's list of interfaces on registration.
Interfaces can be added to a device class at any time. Whenever it is
added, each device in the class is passed to the interface's
add_device callback. When an interface is removed, each device is
removed from the interface.
Devices
~~~~~~~
Once a device is added to a device class, it is added to each
interface that is registered with the device class. The class
is expected to place a class-specific data structure in
struct device::class_data. The interface can use that (along with
other fields of struct device) to determine whether or not the driver
and/or device support that particular interface.
Data
~~~~
struct intf_data {
struct list_head node;
struct device_interface * intf;
struct device * dev;
u32 intf_num;
};
int interface_add_data(struct interface_data *);
The interface is responsible for allocating and initializing a struct
intf_data and calling interface_add_data() to add it to the device's list
of interfaces it belongs to. This list will be iterated over when the device
is removed from the class (instead of all possible interfaces for a class).
This structure should probably be embedded in whatever per-device data
structure the interface is allocating anyway.
Devices are enumerated within the interface. This happens in interface_add_data()
and the enumerated value is stored in the struct intf_data for that device.
sysfs
~~~~~
Each interface is given a directory in the directory of the device
class it belongs to:
Interfaces get a directory in the class's directory as well:
class/
`-- input
|-- devices
|-- drivers
|-- mouse
`-- evdev
When a device is added to the interface, a symlink is created that points
to the device's directory in the physical hierarchy:
class/
`-- input
|-- devices
| `-- 1 -> ../../../root/pci0/00:1f.0/usb_bus/00:1f.2-1:0/
|-- drivers
| `-- usb:usb_mouse -> ../../../bus/drivers/usb_mouse/
|-- mouse
| `-- 1 -> ../../../root/pci0/00:1f.0/usb_bus/00:1f.2-1:0/
`-- evdev
`-- 1 -> ../../../root/pci0/00:1f.0/usb_bus/00:1f.2-1:0/
Future Plans
~~~~~~~~~~~~
A device interface is correlated directly with a userspace interface
for a device, specifically a device node. For instance, a SCSI disk
exposes at least two interfaces to userspace: the standard SCSI disk
interface and the SCSI generic interface. It might also export a raw
device interface.
Many interfaces have a major number associated with them and each
device gets a minor number. Or, multiple interfaces might share one
major number, and each will receive a range of minor numbers (like in
the case of input devices).
These major and minor numbers could be stored in the interface
structure. Major and minor allocations could happen when the interface
is registered with the class, or via a helper function.

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The Linux Kernel Device Model
Patrick Mochel <mochel@osdl.org>
26 August 2002
Overview
~~~~~~~~
This driver model is a unification of all the current, disparate driver models
that are currently in the kernel. It is intended to augment the
bus-specific drivers for bridges and devices by consolidating a set of data
and operations into globally accessible data structures.
Current driver models implement some sort of tree-like structure (sometimes
just a list) for the devices they control. But, there is no linkage between
the different bus types.
A common data structure can provide this linkage with little overhead: when a
bus driver discovers a particular device, it can insert it into the global
tree as well as its local tree. In fact, the local tree becomes just a subset
of the global tree.
Common data fields can also be moved out of the local bus models into the
global model. Some of the manipulations of these fields can also be
consolidated. Most likely, manipulation functions will become a set
of helper functions, which the bus drivers wrap around to include any
bus-specific items.
The common device and bridge interface currently reflects the goals of the
modern PC: namely the ability to do seamless Plug and Play, power management,
and hot plug. (The model dictated by Intel and Microsoft (read: ACPI) ensures
us that any device in the system may fit any of these criteria.)
In reality, not every bus will be able to support such operations. But, most
buses will support a majority of those operations, and all future buses will.
In other words, a bus that doesn't support an operation is the exception,
instead of the other way around.
Downstream Access
~~~~~~~~~~~~~~~~~
Common data fields have been moved out of individual bus layers into a common
data structure. But, these fields must still be accessed by the bus layers,
and sometimes by the device-specific drivers.
Other bus layers are encouraged to do what has been done for the PCI layer.
struct pci_dev now looks like this:
struct pci_dev {
...
struct device device;
};
Note first that it is statically allocated. This means only one allocation on
device discovery. Note also that it is at the _end_ of struct pci_dev. This is
to make people think about what they're doing when switching between the bus
driver and the global driver; and to prevent against mindless casts between
the two.
The PCI bus layer freely accesses the fields of struct device. It knows about
the structure of struct pci_dev, and it should know the structure of struct
device. PCI devices that have been converted generally do not touch the fields
of struct device. More precisely, device-specific drivers should not touch
fields of struct device unless there is a strong compelling reason to do so.
This abstraction is prevention of unnecessary pain during transitional phases.
If the name of the field changes or is removed, then every downstream driver
will break. On the other hand, if only the bus layer (and not the device
layer) accesses struct device, it is only that layer that needs to change.
User Interface
~~~~~~~~~~~~~~
By virtue of having a complete hierarchical view of all the devices in the
system, exporting a complete hierarchical view to userspace becomes relatively
easy. This has been accomplished by implementing a special purpose virtual
file system named sysfs. It is hence possible for the user to mount the
whole sysfs filesystem anywhere in userspace.
This can be done permanently by providing the following entry into the
/etc/fstab (under the provision that the mount point does exist, of course):
none /sys sysfs defaults 0 0
Or by hand on the command line:
# mount -t sysfs sysfs /sys
Whenever a device is inserted into the tree, a directory is created for it.
This directory may be populated at each layer of discovery - the global layer,
the bus layer, or the device layer.
The global layer currently creates two files - 'name' and 'power'. The
former only reports the name of the device. The latter reports the
current power state of the device. It will also be used to set the current
power state.
The bus layer may also create files for the devices it finds while probing the
bus. For example, the PCI layer currently creates 'irq' and 'resource' files
for each PCI device.
A device-specific driver may also export files in its directory to expose
device-specific data or tunable interfaces.
More information about the sysfs directory layout can be found in
the other documents in this directory and in the file
Documentation/filesystems/sysfs.txt.

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Platform Devices and Drivers
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Platform devices
~~~~~~~~~~~~~~~~
Platform devices are devices that typically appear as autonomous
entities in the system. This includes legacy port-based devices and
host bridges to peripheral buses.
Platform drivers
~~~~~~~~~~~~~~~~
Drivers for platform devices are typically very simple and
unstructured. Either the device was present at a particular I/O port
and the driver was loaded, or it was not. There was no possibility
of hotplugging or alternative discovery besides probing at a specific
I/O address and expecting a specific response.
Other Architectures, Modern Firmware, and new Platforms
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
These devices are not always at the legacy I/O ports. This is true on
other architectures and on some modern architectures. In most cases,
the drivers are modified to discover the devices at other well-known
ports for the given platform. However, the firmware in these systems
does usually know where exactly these devices reside, and in some
cases, it's the only way of discovering them.
The Platform Bus
~~~~~~~~~~~~~~~~
A platform bus has been created to deal with these issues. First and
foremost, it groups all the legacy devices under a common bus, and
gives them a common parent if they don't already have one.
But, besides the organizational benefits, the platform bus can also
accommodate firmware-based enumeration.
Device Discovery
~~~~~~~~~~~~~~~~
The platform bus has no concept of probing for devices. Devices
discovery is left up to either the legacy drivers or the
firmware. These entities are expected to notify the platform of
devices that it discovers via the bus's add() callback:
platform_bus.add(parent,bus_id).
Bus IDs
~~~~~~~
Bus IDs are the canonical names for the devices. There is no globally
standard addressing mechanism for legacy devices. In the IA-32 world,
we have Pnp IDs to use, as well as the legacy I/O ports. However,
neither tell what the device really is or have any meaning on other
platforms.
Since both PnP IDs and the legacy I/O ports (and other standard I/O
ports for specific devices) have a 1:1 mapping, we map the
platform-specific name or identifier to a generic name (at least
within the scope of the kernel).
For example, a serial driver might find a device at I/O 0x3f8. The
ACPI firmware might also discover a device with PnP ID (_HID)
PNP0501. Both correspond to the same device and should be mapped to the
canonical name 'serial'.
The bus_id field should be a concatenation of the canonical name and
the instance of that type of device. For example, the device at I/O
port 0x3f8 should have a bus_id of "serial0". This places the
responsibility of enumerating devices of a particular type up to the
discovery mechanism. But, they are the entity that should know best
(as opposed to the platform bus driver).
Drivers
~~~~~~~
Drivers for platform devices should have a name that is the same as
the canonical name of the devices they support. This allows the
platform bus driver to do simple matching with the basic data
structures to determine if a driver supports a certain device.
For example, a legacy serial driver should have a name of 'serial' and
register itself with the platform bus.
Driver Binding
~~~~~~~~~~~~~~
Legacy drivers assume they are bound to the device once they start up
and probe an I/O port. Divorcing them from this will be a difficult
process. However, that shouldn't prevent us from implementing
firmware-based enumeration.
The firmware should notify the platform bus about devices before the
legacy drivers have had a chance to load. Once the drivers are loaded,
they driver model core will attempt to bind the driver to any
previously-discovered devices. Once that has happened, it will be free
to discover any other devices it pleases.

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Porting Drivers to the New Driver Model
Patrick Mochel
7 January 2003
Overview
Please refer to Documentation/driver-model/*.txt for definitions of
various driver types and concepts.
Most of the work of porting devices drivers to the new model happens
at the bus driver layer. This was intentional, to minimize the
negative effect on kernel drivers, and to allow a gradual transition
of bus drivers.
In a nutshell, the driver model consists of a set of objects that can
be embedded in larger, bus-specific objects. Fields in these generic
objects can replace fields in the bus-specific objects.
The generic objects must be registered with the driver model core. By
doing so, they will exported via the sysfs filesystem. sysfs can be
mounted by doing
# mount -t sysfs sysfs /sys
The Process
Step 0: Read include/linux/device.h for object and function definitions.
Step 1: Registering the bus driver.
- Define a struct bus_type for the bus driver.
struct bus_type pci_bus_type = {
.name = "pci",
};
- Register the bus type.
This should be done in the initialization function for the bus type,
which is usually the module_init(), or equivalent, function.
static int __init pci_driver_init(void)
{
return bus_register(&pci_bus_type);
}
subsys_initcall(pci_driver_init);
The bus type may be unregistered (if the bus driver may be compiled
as a module) by doing:
bus_unregister(&pci_bus_type);
- Export the bus type for others to use.
Other code may wish to reference the bus type, so declare it in a
shared header file and export the symbol.
From include/linux/pci.h:
extern struct bus_type pci_bus_type;
From file the above code appears in:
EXPORT_SYMBOL(pci_bus_type);
- This will cause the bus to show up in /sys/bus/pci/ with two
subdirectories: 'devices' and 'drivers'.
# tree -d /sys/bus/pci/
/sys/bus/pci/
|-- devices
`-- drivers
Step 2: Registering Devices.
struct device represents a single device. It mainly contains metadata
describing the relationship the device has to other entities.
- Embedd a struct device in the bus-specific device type.
struct pci_dev {
...
struct device dev; /* Generic device interface */
...
};
It is recommended that the generic device not be the first item in
the struct to discourage programmers from doing mindless casts
between the object types. Instead macros, or inline functions,
should be created to convert from the generic object type.
#define to_pci_dev(n) container_of(n, struct pci_dev, dev)
or
static inline struct pci_dev * to_pci_dev(struct kobject * kobj)
{
return container_of(n, struct pci_dev, dev);
}
This allows the compiler to verify type-safety of the operations
that are performed (which is Good).
- Initialize the device on registration.
When devices are discovered or registered with the bus type, the
bus driver should initialize the generic device. The most important
things to initialize are the bus_id, parent, and bus fields.
The bus_id is an ASCII string that contains the device's address on
the bus. The format of this string is bus-specific. This is
necessary for representing devices in sysfs.
parent is the physical parent of the device. It is important that
the bus driver sets this field correctly.
The driver model maintains an ordered list of devices that it uses
for power management. This list must be in order to guarantee that
devices are shutdown before their physical parents, and vice versa.
The order of this list is determined by the parent of registered
devices.
Also, the location of the device's sysfs directory depends on a
device's parent. sysfs exports a directory structure that mirrors
the device hierarchy. Accurately setting the parent guarantees that
sysfs will accurately represent the hierarchy.
The device's bus field is a pointer to the bus type the device
belongs to. This should be set to the bus_type that was declared
and initialized before.
Optionally, the bus driver may set the device's name and release
fields.
The name field is an ASCII string describing the device, like
"ATI Technologies Inc Radeon QD"
The release field is a callback that the driver model core calls
when the device has been removed, and all references to it have
been released. More on this in a moment.
- Register the device.
Once the generic device has been initialized, it can be registered
with the driver model core by doing:
device_register(&dev->dev);
It can later be unregistered by doing:
device_unregister(&dev->dev);
This should happen on buses that support hotpluggable devices.
If a bus driver unregisters a device, it should not immediately free
it. It should instead wait for the driver model core to call the
device's release method, then free the bus-specific object.
(There may be other code that is currently referencing the device
structure, and it would be rude to free the device while that is
happening).
When the device is registered, a directory in sysfs is created.
The PCI tree in sysfs looks like:
/sys/devices/pci0/
|-- 00:00.0
|-- 00:01.0
| `-- 01:00.0
|-- 00:02.0
| `-- 02:1f.0
| `-- 03:00.0
|-- 00:1e.0
| `-- 04:04.0
|-- 00:1f.0
|-- 00:1f.1
| |-- ide0
| | |-- 0.0
| | `-- 0.1
| `-- ide1
| `-- 1.0
|-- 00:1f.2
|-- 00:1f.3
`-- 00:1f.5
Also, symlinks are created in the bus's 'devices' directory
that point to the device's directory in the physical hierarchy.
/sys/bus/pci/devices/
|-- 00:00.0 -> ../../../devices/pci0/00:00.0
|-- 00:01.0 -> ../../../devices/pci0/00:01.0
|-- 00:02.0 -> ../../../devices/pci0/00:02.0
|-- 00:1e.0 -> ../../../devices/pci0/00:1e.0
|-- 00:1f.0 -> ../../../devices/pci0/00:1f.0
|-- 00:1f.1 -> ../../../devices/pci0/00:1f.1
|-- 00:1f.2 -> ../../../devices/pci0/00:1f.2
|-- 00:1f.3 -> ../../../devices/pci0/00:1f.3
|-- 00:1f.5 -> ../../../devices/pci0/00:1f.5
|-- 01:00.0 -> ../../../devices/pci0/00:01.0/01:00.0
|-- 02:1f.0 -> ../../../devices/pci0/00:02.0/02:1f.0
|-- 03:00.0 -> ../../../devices/pci0/00:02.0/02:1f.0/03:00.0
`-- 04:04.0 -> ../../../devices/pci0/00:1e.0/04:04.0
Step 3: Registering Drivers.
struct device_driver is a simple driver structure that contains a set
of operations that the driver model core may call.
- Embed a struct device_driver in the bus-specific driver.
Just like with devices, do something like:
struct pci_driver {
...
struct device_driver driver;
};
- Initialize the generic driver structure.
When the driver registers with the bus (e.g. doing pci_register_driver()),
initialize the necessary fields of the driver: the name and bus
fields.
- Register the driver.
After the generic driver has been initialized, call
driver_register(&drv->driver);
to register the driver with the core.
When the driver is unregistered from the bus, unregister it from the
core by doing:
driver_unregister(&drv->driver);
Note that this will block until all references to the driver have
gone away. Normally, there will not be any.
- Sysfs representation.
Drivers are exported via sysfs in their bus's 'driver's directory.
For example:
/sys/bus/pci/drivers/
|-- 3c59x
|-- Ensoniq AudioPCI
|-- agpgart-amdk7
|-- e100
`-- serial
Step 4: Define Generic Methods for Drivers.
struct device_driver defines a set of operations that the driver model
core calls. Most of these operations are probably similar to
operations the bus already defines for drivers, but taking different
parameters.
It would be difficult and tedious to force every driver on a bus to
simultaneously convert their drivers to generic format. Instead, the
bus driver should define single instances of the generic methods that
forward call to the bus-specific drivers. For instance:
static int pci_device_remove(struct device * dev)
{
struct pci_dev * pci_dev = to_pci_dev(dev);
struct pci_driver * drv = pci_dev->driver;
if (drv) {
if (drv->remove)
drv->remove(pci_dev);
pci_dev->driver = NULL;
}
return 0;
}
The generic driver should be initialized with these methods before it
is registered.
/* initialize common driver fields */
drv->driver.name = drv->name;
drv->driver.bus = &pci_bus_type;
drv->driver.probe = pci_device_probe;
drv->driver.resume = pci_device_resume;
drv->driver.suspend = pci_device_suspend;
drv->driver.remove = pci_device_remove;
/* register with core */
driver_register(&drv->driver);
Ideally, the bus should only initialize the fields if they are not
already set. This allows the drivers to implement their own generic
methods.
Step 5: Support generic driver binding.
The model assumes that a device or driver can be dynamically
registered with the bus at any time. When registration happens,
devices must be bound to a driver, or drivers must be bound to all
devices that it supports.
A driver typically contains a list of device IDs that it supports. The
bus driver compares these IDs to the IDs of devices registered with it.
The format of the device IDs, and the semantics for comparing them are
bus-specific, so the generic model does attempt to generalize them.
Instead, a bus may supply a method in struct bus_type that does the
comparison:
int (*match)(struct device * dev, struct device_driver * drv);
match should return '1' if the driver supports the device, and '0'
otherwise.
When a device is registered, the bus's list of drivers is iterated
over. bus->match() is called for each one until a match is found.
When a driver is registered, the bus's list of devices is iterated
over. bus->match() is called for each device that is not already
claimed by a driver.
When a device is successfully bound to a device, device->driver is
set, the device is added to a per-driver list of devices, and a
symlink is created in the driver's sysfs directory that points to the
device's physical directory:
/sys/bus/pci/drivers/
|-- 3c59x
| `-- 00:0b.0 -> ../../../../devices/pci0/00:0b.0
|-- Ensoniq AudioPCI
|-- agpgart-amdk7
| `-- 00:00.0 -> ../../../../devices/pci0/00:00.0
|-- e100
| `-- 00:0c.0 -> ../../../../devices/pci0/00:0c.0
`-- serial
This driver binding should replace the existing driver binding
mechanism the bus currently uses.
Step 6: Supply a hotplug callback.
Whenever a device is registered with the driver model core, the
userspace program /sbin/hotplug is called to notify userspace.
Users can define actions to perform when a device is inserted or
removed.
The driver model core passes several arguments to userspace via
environment variables, including
- ACTION: set to 'add' or 'remove'
- DEVPATH: set to the device's physical path in sysfs.
A bus driver may also supply additional parameters for userspace to
consume. To do this, a bus must implement the 'hotplug' method in
struct bus_type:
int (*hotplug) (struct device *dev, char **envp,
int num_envp, char *buffer, int buffer_size);
This is called immediately before /sbin/hotplug is executed.
Step 7: Cleaning up the bus driver.
The generic bus, device, and driver structures provide several fields
that can replace those defined privately to the bus driver.
- Device list.
struct bus_type contains a list of all devices registered with the bus
type. This includes all devices on all instances of that bus type.
An internal list that the bus uses may be removed, in favor of using
this one.
The core provides an iterator to access these devices.
int bus_for_each_dev(struct bus_type * bus, struct device * start,
void * data, int (*fn)(struct device *, void *));
- Driver list.
struct bus_type also contains a list of all drivers registered with
it. An internal list of drivers that the bus driver maintains may
be removed in favor of using the generic one.
The drivers may be iterated over, like devices:
int bus_for_each_drv(struct bus_type * bus, struct device_driver * start,
void * data, int (*fn)(struct device_driver *, void *));
Please see drivers/base/bus.c for more information.
- rwsem
struct bus_type contains an rwsem that protects all core accesses to
the device and driver lists. This can be used by the bus driver
internally, and should be used when accessing the device or driver
lists the bus maintains.
- Device and driver fields.
Some of the fields in struct device and struct device_driver duplicate
fields in the bus-specific representations of these objects. Feel free
to remove the bus-specific ones and favor the generic ones. Note
though, that this will likely mean fixing up all the drivers that
reference the bus-specific fields (though those should all be 1-line
changes).